The invention relates to a liquid crystal display device comprising a first substrate which is provided with electrodes, and a second substrate which is parallel to the first substrate and is provided with electrodes, and a twisted nematic liquid crystal material having a positive dielectric anisotropy between the two substrates, while, viewed perpendicularly to the substrates, overlapping parts of the electrodes define pixels, the display device being further provided with drive means for presenting voltages to the electrodes.
Display devices of this type are generally known and are used, for example in display screens for alphanumerical display devices in, for example, computer apparatus and measuring apparatus, but also in car radios and telephone apparatus.
The use of such display devices may give rise to problems at a varying ambient temperature, because values which are characteristic of the liquid crystal, such as threshold voltage and saturation voltage, are dependent on temperature. To be able to use these display devices in a large temperature range, the drive voltages are usually adapted in dependence upon the temperature, as described, for example, in U.S. Pat. No. 4,298,866. However, this means that the drive voltages are chosen from a fairly large voltage range, which requires a high power supply voltage for the drive electronics. Notably in portable apparatus, as in said measuring and telephone apparatus which are usually battery-powered, this may give rise to problems.
This problem may be partly solved by choosing a liquid crystal material having a low threshold voltage and thus a lower operating voltage. Such low threshold voltages (and hence the other characteristic voltages of the display elements) have the additional advantage that a higher multiplex ratio can be achieved in the multiplex drive mode.
A drawback is that the threshold voltage is related to the dielectric constant of the liquid crystal material parallel to the liquid crystal molecules (∈¦¦). When the threshold voltage decreases, ∈¦¦ increases.
Said dielectric constant is frequency-dependent, which means in practice that, above a given threshold frequency, this dielectric constant decreases due to relaxation so that the threshold voltage of the liquid crystal material effectively increases above this threshold frequency. To be able to realize the desired low threshold voltage, the drive frequency is therefore chosen in practice as low as possible. On the other hand, a pixel must be driven at a sufficiently high frequency so as to prevent flicker (the repetition time should be several times (about a factor of 5) smaller than the response time of the liquid crystal material.
It is an object of the invention to substantially obviate one or more of the above-mentioned problems. It is another object of the invention to provide a display device, notably based on the STN effect (twist angles between 160xc2x0 and 360xc2x0) having a high multiplex degree through a wide range of temperatures.
To this end, a liquid crystal display device according to the invention is characterized in that the drive means are provided with means for controlling the frequency at which pixels are driven, dependent on the temperature of the display device.
The invention is based on the recognition that, notably upon a decrease of temperature, the threshold voltage (and hence the other characteristic voltages of the display elements) strongly increase with the frequency, while much lower drive frequencies can be used at these lower temperatures. This particularly applies to said liquid crystal materials having a low threshold voltage (Vth less than 2 Voltsxcx9c, particularly less than 1,8 Volts). The relevant frequency control does not necessarily have to work for a full range of temperatures but may be limited to, for example, a range of temperatures below 20xc2x0 C.
A preferred embodiment of a display device is characterized in that the drive means comprise means for decreasing the frequency at which pixels are driven when the temperature decreases, and for increasing the frequency when the temperature increases. The frequency adaptation is not necessarily continuous but may be performed in steps. In that case, the drive means are provided, for example, with an oscillator and a plurality of frequency dividers which adjust the frequency in dependence on the temperature value supplied by a temperature sensor.
A further preferred embodiment is characterized in that the drive means are provided with means for increasing the duration of row selection times when the frequency at which the rows are selected is decreased, and for decreasing the duration of row selection times when the frequency at which the rows are selected is increased. Notably in the RMS drive mode, it is then achieved that the effective voltage across the pixels can be applied at lower data and selection voltages.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.